Typical anti-lock brake systems are able to prevent a vehicle wheel from completely locking up, or stopping rotation, while the vehicle is still in motion.
Pressure to a wheel brake is selectively interrupted to prevent lock up of the wheel. A first valve positioned in an inlet conduit between a master cylinder and the wheel brake is closed. Residual pressure within the wheel brake is released by opening a second valve positioned in an outlet conduit which is fluidly connected with the wheel brake. The first valve is then opened and the second valve closed to restore pressure to the wheel brake, with the valves closing and opening again respectively to relieve pressure within the brake if the wheel begins to lock up again. This on-and-off cycling continues until the vehicle either comes to a stop or is traveling at a speed below a preestablished threshold limit.
Fluid is displaced from the brake during the release portion of the cycle. The displaced fluid is directed through the outlet conduit to a pump. The pump pressurizes the fluid and returns it to the inlet conduit. The pump has a motor driven eccentric which displaces a pump piston to pressurize the fluid. The motion of the piston produced by the eccentric rapidly pressurizes the hydraulic brake fluid in the pump, thereby introducing high pressure build gradients. This rapid pressurization, and the subsequent return of fluid to the inlet conduit, is believed to cause undesirable noise, deemed "fluid hammering," in the anti-lock brake system. Several methods attempt to reduce fluid hammering, however, these methods have several shortcoming and drawbacks.
One attempt at reducing fluid hammering consists of inserting a damping chamber or accumulator in the anti-lock braking system on an output or pressure side of the pump. The damping chamber is designed to reduce the high pressure build gradients, and thus reduce the fluid hammering. Although the use of damping chambers does aid in reducing fluid hammering, damping chambers fall short of eliminating the undesirable fluid hammering from the system.
Another attempt at reducing fluid hammering focuses on the internal workings of the pump itself. This approach consists of boring out an axial cavity in an end of the pump piston. The resulting cavity is filled with a rubber plug, a Teflon.TM. spacer and a steel insert. Rotation of the eccentric by the motor imparts motion to the steel insert, which transfers motion to the Teflon.TM. spacer which transfers the motion to the rubber plug which finally transfers the motion to the pump piston. The combination of the rubber plug, the Teflon.TM., and the steel insert dampens the transfer of motion from the eccentric to the piston, thereby reducing the undesirable fluid hammering. This set of elements for reducing fluid hammering exhibits several limitations. First, the bearing area of the steel insert against the eccentric is only as large as the diameter of the axial cavity. The use of such a relatively small area as a bearing surface introduces a potential durability concern. Second, because of their small size, the manipulation and insertion into the piston of the rubber plug, Teflon.TM. spacer and steel insert is difficult. Lastly, the reduction in fluid hammering provided by the rubber plug is limited in part because it is trapped in a space substantially equal in shape and volume to itself. The plug is thus unable to appreciably deform in response to compressive forces to provide the desired damping effect.